The present invention refers to a fuel/air mixture preparation device for internal combustion engines, the device having a nozzle body of rotational symmetry which, together with a throttle element of rotational symmetry which is displaceable within the body, forms a convergent-divergent nozzle directed into an intake tube of the internal combustion engine, and wherein the device includes at least one fuel feed line which opens into the nozzle in the vicinity of a narrowest cross section of the nozzle.
With such a known fuel/air mixture preparation device, it is endeavored optimally to control both the atomization of the fuel and the mass throughput of the intake air over operating range of the internal combustion engine which is fed this fuel/air mixture. This is achieved in accordance with the principle that liquid fuel be introduced into the intake air stream and distributed as uniformly as possible with a stream of air flowing flows through a constricted cross section of the mixture preparation device wherein, the speed of the air increases to above the speed of sound. The fuel is thereby divided into fine droplets The free cross sectional area of the stream of air between the nozzle body and a axially adjustable throttle element can also be changed similarly, the amount of fuel introduced can be controlled.
Downstream of the narrowest cross section in the nozzle body, the stream of air and the particles of fuel contained therein are accelerated to supersonic speed; This mixture is then again retarded in an impact zone to a lower speed, lying below the speed of sound, before the fuel/air mixture passes into the cylinders of the combustion engine. The space within this nozzle body is limited by a wall which initially tapers down in the direction of the stream of air drawn in and then, starting at the narrowest cross sectional place, again widens in a so-called diffuser Both the nozzle body and the throttle element are developed in this connection with substantially rotational symmetry about a longitudinal axis which also represents the main direction of flow of the air drawn in.
This aspect of the mixture preparation device, causes an asymmetric distribution of the fuel in the air drawn in, as occurs generally in carburetors with pivoted throttle valves is to be avoided. Furthermore, as compared with well-known carburetors, there is a disadvantage in that the composition of the fuel/air mixture is subjected to strong variations as a function of the flow of air through the carburetor. This disadvantage is to be solved by the invention.
In connection herewith, it is undesirable that the fuel/air mixture be, on the one hand, too rich so that the fuel cannot burn completely in the cylinders or, on the other hand, too lean, as a result of which misfiring can take place. The excessively rich mixture not only reduces the efficiency of the internal combustion engine but also results, in particular, in an increased emission of contaminating substances.
A substantial factor for avoidance of the disadvantages of prior art carburetors with pivoted throttle valve is attained by a feeding of fuel into a space which is limited towards the inside by a wall of the throttle element of rotational symmetry. In principle, liquid fuel is to be introduced via conduits into an inwardly drawn stream of air above the narrowest cross sectional place of the nozzle body into the latter. For this purpose, fuel lines are introduced through the wall of the nozzle body into its (inner) space.
In one known variant of this mixture preparation device with nozzle body of rotational symmetry, the feeding of the fuel is effected via a fuel nozzle which is located above the throttle element along the longitudinal axis of the nozzle body of rotational symmetry. The air-intake fuel nozzle is fed with air which is under pressure as well as with a controlled flow of fuel. The opening of the nozzle faces a baffle plate by which the fuel is to be injected radially in substantially symmetrical distribution into the space within the nozzle. The nozzle is in this connection developed as air-intake nozzle. The liquid fuel which is sprayed by this nozzle reaches the inner surface of the wall in the nozzle body and travels down along the inclined wall to the place of narrowest cross section. In this connection, the downward flowing quantity of fuel is to be substantially uniformly distributed over the circumference of the wall. In the region of the place of narrowest cross section, the fuel is detached from the wall by the air which is flowing with high speed and is distributed in fine form in the air.
In another known embodiment, the feeding of compressed air to the nozzle is dispensed with. In furtherance of the last-mentioned principle, the feeding of the fuel has been so modified that it takes place through a conduit over an annular body into which the nozzle body of rotational symmetry is inserted, which nozzle body tapers down on the inside down to its place of narrowest cross section. Between the annular body and the nozzle body there is thus formed a cylindrical slot, the slot opening of which is limited by the upper edge of the nozzle body.
Thus the fuel conveyed through this cylindrical fuel slot travels over the upper edge of the nozzle body over its entire length, down to the place of narrowest cross section where, in the manner indicated, it is to be detached by air flowing at high speed from the inner surface of the wall of the nozzle body to be atomized. As a result of the adherence forces between the film of fuel and the wall, only a part of the fuel, however, is actually atomized. In this development of the device for the preparation of the mixture, changes in the metering of the fuel furthermore act only with delay on the fuel/air mixture formed, since the fuel must first of all flow down on the inner surface of the wall of the nozzle body until it passes, substantially at the point of narrowest cross section, into the fuel/air mixture. In other words, changes in pressure and/or flow in the fuel line do not result directly in suitable preparation of the mixture since the volume of fuel can spread out outside the annular body.